Chip packaging process

ABSTRACT

A chip packaging process is provided. First, a cavity is formed on a heat sink. A first encapsulant is formed on the bottom of the cavity. A circuit substrate is disposed over the heat sink. The circuit substrate has an opening that corresponds in position to the cavity. Thereafter, a chip is disposed on the first encapsulant and the chip is electrically connected to the circuit substrate. Finally, a compound is deposited over the first encapsulant and the chip to form a chip package. The chip package is warp resistant and the chip packaging process increases overall production yield.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of an application Ser. No. 10/907,340,filed on Mar. 30, 2005, now allowed, which claims the priority benefitof Taiwan application serial no. 93109186, filed on Apr. 2, 2004. Theentirety of each of the above-mentioned patent applications is herebyincorporated by reference herein and made a part of this specification.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a chip package structure and a chippackaging process. More particularly, the present invention relates to awarp-resistant chip package structure and chip packaging process forimproving the yield of production.

2. Description of the Related Art

With the rapid progress in electronic technologies and semiconductormanufacturing in recent years, a lot of personalized and multifunctionalelectronic products flooded the market. Moreover, these electronicproducts are often miniaturized and downsized for portability. In thearea of semiconductor production, the fabrication of integrated circuits(IC) can be roughly divided into three main stages: integrated circuitdesign, integrated circuit fabrication and integrated circuit packaging.In general, raw chips are produced after going through a series of stepsincluding wafer production, circuit design, mask production and waferdicing operation. Each piece of raw chip cut out from the wafer must beelectrically connected to a substrate for external communication throughthe bonding pads thereon. Moreover, each raw chip must be encapsulatedby an encapsulant to form an IC package that prevents moisture, heat orelectrical noise from interfering with its function and a medium forconnecting with an external circuit such as a printed circuit board(PCB) or other packaging substrate.

Due to the rapid advance in IC production technologies, the operatingspeed of each chip continues to increase so that various types ofdigital data processing and computation are increasingly efficient.However, as the level of integration of circuits within each chipincreases exponentially, the amount of heat generated per unit area innormal operation also increases accordingly. At maximum operating speed,the need to dissipate the heat away from the chip fast enough iscritical. Conventionally, a metallic heat sink with highheat-dissipating capacity is disposed on a chip package to absorb heatand conduct the heat away to the surrounding atmosphere.

FIG. 1 is a schematic cross-sectional view showing the structure of aconventional chip package. As shown in FIG. 1, the chip packagestructure 100 comprises a chip 110, a circuit substrate 120, a heat sink130 and an encapsulant 140. The circuit substrate 120 having an opening120 a therein is disposed on the heat sink 130. The back surface 110 bof the chip 110 is mounted to the heat sink 130 within the opening 120 aof the circuit substrate 120 by adhesive glue (not shown). The activesurface 110 a of the chip 110 has a plurality of bonding pads 112thereon. The circuit substrate 120 has a plurality of first bonding pads122 a and a plurality of second bonding pads 122 b surrounding the firstbonding pads 122 a. The chip 110 and the circuit board 120 areelectrically connected through a wire-bonding process. Typically, eachbonding pad 112 on the chip 110 is electrically connected to acorresponding first bonding pad 122 a on the circuit substrate 120through a conductive wire 150.

As shown in FIG. 1, the encapsulant 140 fills up the opening 120 aentirely and encapsulates the chip 110, the conductive wires 150 and thefirst bonding pads 122 a on the circuit substrate 120. The encapsulant140 prevents moisture, heat or electrical noise from affecting the chip110 and protects the conductive wires 150 against mechanical damage.Furthermore, a plurality of conductive bumps 160 is formed on the secondbonding pads 122 b of the circuit substrate 120 to serve as a medium ofcontact for connecting the chip package 100 to external circuits.

To form the encapsulant in the aforementioned chip packaging process, amolding compound such as epoxy resin is heated to a semi-liquid form ata high temperature and injected into a mold. Thereafter, the moldingcompound is cooled to form a solidified encapsulant. However, the heatsink and the encapsulant have different coefficient of thermal expansion(CTE). Thus, a thermal stress of non-uniform magnitude is oftengenerated inside the chip package somewhere between the heat sink andthe encapsulant during the cooling process. Due to the thermal stress,the chip package is more likely to warp or crack. The warping orcracking of the chip package frequently leads to irreversible damage toeither the chip or the conductive wires. Therefore, the yield of thechip packaging process can not be enhanced effectively.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to provide a chip packagestructure, wherein the thermal stress around a chip is uniform. Thus,warping and cracking of the chip package is significantly reduced andthe reliability of the connection between the chip and conductive wiresinside the chip package is maintained.

The prevent invention is further directed to provide a chip packagingprocess that includes the step of encapsulating a chip with an identicalmaterial so that the effect of thermal stress on the chip package issubstantially reduced. Consequently, there is a significant improvementin overall product yield of the chip package.

According to an embodiment of the present invention, a chip packagestructure is provided. The chip package comprises a heat sink, a circuitsubstrate, a chip, a first encapsulant, a second encapsulant and aplurality of conductive bumps. The heat sink has a cavity and thecircuit substrate is disposed on the heat sink. The circuit substratehas a top surface and an opening that corresponds in position to thecavity. The chip is disposed at the bottom of the cavity and iselectrically connected to the circuit substrate. The first encapsulantis disposed between the heat sink and the chip. Furthermore, the firstencapsulant is in contact with the sidewall of the cavity. The secondencapsulant is disposed over the first encapsulant to cover the chip anda portion of the circuit substrate. The conductive bumps are disposed onthe top surface of the circuit substrate outside the second encapsulantto serve as a medium of contact for connecting the chip package withexternal circuits.

In an embodiment of the present invention, the first encapsulant has aheight small than, equal to or greater than the depth of the cavity. Thechip package structure may further comprise a heat-conductive interlayersuch as a metallic layer or a patterned metallic layer. Theheat-conductive interlayer is disposed between the chip and the firstencapsulant and extends between the circuit substrate and the heat sinkto conduct heat directly away to the heat sink. The chip and the circuitsubstrate are electrically connected through a plurality of conductivewires bonded to the circuit substrate and the chip in a wire-bondingprocess. The second encapsulant further encapsulates the conductivewires to protect the conductive wires against possible damage resultingfrom external forces.

In one embodiment of the present invention, the first encapsulant andthe second encapsulant are fabricated using an identical material suchas epoxy resin. In addition, the conductive bumps are distributed aroundthe second encapsulant.

The present invention further provides an alternative chip packagestructure. The chip package comprises a heat sink, a circuit substrate,a chip, an encapsulant and a plurality of conductive bumps. The heatsink has a cavity and the circuit board is disposed on the heat sink.The circuit board has a top surface and an opening that corresponds inposition to the cavity. The chip is disposed in the cavity. The chip andthe circuit substrate are electrically connected through a plurality ofconductive wires. The encapsulant encapsulates the chip and theconductive wires and is in contact with the sidewalls and bottom surfaceof the cavity. The conductive bumps are disposed on the top surface ofthe circuit substrate.

The present invention further provides a chip packaging processcomprising the following steps. First, a cavity is formed on a heat sinkand then a first encapsulant is formed at the bottom of the cavity. Acircuit substrate is disposed on the heat sink. The circuit substratehas an opening that corresponds in position to the cavity. A chip isdisposed on the first encapsulant. The chip and the circuit substrateare electrically connected. Thereafter, a second encapsulant is formedover the first encapsulant to encapsulate the chip and a portion of thecircuit substrate. Ultimately, a plurality of conductive bumps areformed on the circuit substrate.

In one embodiment of the present invention, the chip packaging processmay include forming a heat-conductive interlayer over the firstencapsulant and the heat sink before disposing the circuit substrateover the heat sink. Thus, a more efficient heat conductive path iscreated. In addition, the heat-conductive interlayer can be a metalliclayer or a patterned metallic layer. Moreover, the chip and the circuitsubstrate are electrically connected through conductive wires formed ina wire-bonding process.

In brief, the chip package structure and the chip packaging process ischaracterized by disposing a first encapsulant between the chip and theheat sink to increase the gap formed between the chip and the heat sink.The first encapsulant and the second encapsulant are fabricated from anidentical material so that they have an identical coefficient of thermalexpansion. Hence, the degree of warping in the chip package is minimizedwhen the molding compound for forming the second encapsulant is cooled.Furthermore, since the chip is entirely encapsulated by the firstencapsulant and the second encapsulant, a uniform thermal stress isgenerated around the chip. As a result, reliability of the connectionbetween the chip and the conductive wires is maintained and the chippackaging process with a higher overall yield is provided.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary, and are intended toprovide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate embodiments of theinvention and, together with the description, serve to explain theprinciples of the invention.

FIG. 1 is a schematic cross-sectional view showing the structure of aconventional chip package.

FIGS. 2A through 2G are schematic cross-sectional view showing the stepsin a chip packaging process for producing a chip package according to afirst embodiment of the present invention.

FIG. 3 is a schematic cross-sectional view of a second chip packagestructure fabricated using the chip packaging process according to thefirst embodiment of the present invention.

FIG. 4 is a schematic cross-sectional view of a third chip packagestructure fabricated using the chip packaging process according to thefirst embodiment of the present invention.

FIG. 5 is a schematic cross-sectional view of a first chip packagestructure according to a second embodiment of the present invention.

FIG. 6 is a schematic cross-sectional view of a second chip packagestructure according to the second embodiment of the present invention.

FIGS. 7A and 7B are graphs obtained from a simulation showing the degreeof warping between the chip package of the present invention and theconventional chip package at different temperatures.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the present embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers are used in thedrawings and the description to refer to the same or like parts.

FIGS. 2A through 2G are schematic cross-sectional view showing the stepsin a chip packaging process for producing a chip package according to afirst embodiment of the present invention. First, as shown in FIG. 2A, acavity 230 a is formed on a heat sink 230. The cavity 230 a is formed,for example, by performing mechanical drilling, laser drilling oretching. As shown in FIG. 2B, a first encapsulant 240 a is formed on thebottom of the cavity 230 a by applying a material such as epoxy resin.Thereafter, as shown in FIG. 2C, a circuit substrate 220 is disposed onthe heat sink 230. The circuit substrate 220 has an opening 220 a thatcorresponds in position to the cavity 230 a. Furthermore, the circuitsubstrate 220 has a top surface 224 a having a plurality of firstbonding pads 222 a and a plurality of second bonding pads 222 b thereon.The second bonding pads 222 b are located around the first bonding pads222 a. The bottom surface 224 b of the circuit substrate 220 is attachedto the heat sink 230 by adhesive glue (not shown), for example.

As shown in FIG. 2D, a chip 210 is disposed on the first encapsulant 240a. The chip 210 has an active surface 210 a and a back surface 210 b.The active surface 210 a has a plurality of bonding pads 212 thereon. Asshown in FIG. 2E, a wire-bonding process is performed to electricallyconnect the bonding pads 212 on the chip 210 with the first bonding pads222 a on the circuit substrate 220 by a plurality of conductive wires250. As shown in FIG. 2F, a second encapsulant 240 b is formed over thefirst encapsulant 240 a. The second encapsulant 240 b is fabricated froma material identical to the first encapsulant 240 a (such as epoxyresin). The second encapsulant 240 b encapsulates the first bonding pads222 a, the conductive lines 250 and the chip 210 (including the bondingpads 212). Finally, as shown in FIG. 2G, a plurality of conductive bumps260 are formed on the second bonding pads 222 b of the circuit substrate220. After the conductive bumps 260 are formed, a complete chip package200 is provided. The conductive bumps 260 has a height greater than theheight of the second encapsulant 240 b above the top surface 224 a ofthe circuit substrate 220 to facilitate electrically connecting the chippackage 200 through the conductive bumps 260 with an external device.

In the present embodiment, the chip package structure and the chippackaging process use identical material (including the secondencapsulant 240 b and the first encapsulant 240 a) to encapsulate thechip. The material around the chip serves as a buffer to reduce thedegree of warping in the chip package, maintains a fixed relationshipbetween the chip and the conductive wires and increases product yield ofthe chip packaging process. However, the chip packaging process of thepresent invention further permits the disposition of the circuitsubstrate 220 over the heat sink 230 before forming the firstencapsulant 240 a inside the cavity 230 a for supporting the chip 210.Since the process has been explained with reference to FIGS. 2B and 2C,detailed description is omitted.

In addition, both the first encapsulant 240 a and the second encapsulant240 b may be fabricated from an identical material as shown in FIG. 2G.Furthermore, since the chip 210 may be located close to the center ofthe cavity 230 a when the height H1 of the first encapsulant 240 a andthe height H2 of the second encapsulant 210 b are near, a uniform stressaround the chip 210 is obtained. In other words, the chip 210 is moreresistance to warping as a result of thermal stresses. However, theheight H1 of the first encapsulant 240 a need not be close to theheights H2 of the second encapsulant 230 b as shown in FIG. 2G. Inpractice, different types of molding compounds can be used to match theparticular type of design to reduce production cost.

FIG. 3 is a schematic cross-sectional view of another chip packagestructure fabricated using the chip packaging process according to thefirst embodiment of the present invention. As shown in FIG. 3, the topsurface of the first encapsulant 240 a is at a level lower than thebottom surface 224 b of the circuit substrate 220. FIG. 4 is a schematiccross-sectional view of still another chip package structure fabricatedusing the chip packaging process according to the first embodiment ofthe present invention. As shown in FIG. 4, the circuit substrate 220 isformed over the heat sink before forming the first encapsulant 240 a sothat the top surface of the first encapsulant 240 a can reach a heightbetween the top surface 224 a and the bottom surface 224 b of thecircuit substrate 220.

Using the first type of chip package structure and its correspondingchip packaging process as a base, a chip package structure and chippackaging process having a higher heat dissipation can be produced. FIG.5 is a schematic cross-sectional view of a first chip package structureaccording to a second embodiment of the present invention. As shown inFIGS. 2G and 5, the chip package structure 300 in FIG. 5 has anadditional heat-conductive interlayer 270 disposed between the chip 210and the first encapsulant 240 a. The heat-conductive interlayer 270 isfabricated by silver, copper or some other heat conductive metallicmaterials. Furthermore, the heat-dissipation layer 270 may extendbetween the circuit substrate 220 and the heat sink 230 so that heatgenerated by the chip 210 can be directly transferred to the heat sink230 via the heat-conductive interlayer 270.

The chip packaging process for forming the chip package structure 300 inFIG. 5 includes performing an electroplating to form the heat-conductiveinterlayer 270 after forming the first encapsulant 240 a (as shown inFIG. 2B). The process may also include a patterning step to form apatterned heat-conductive interlayer 270. FIG. 6 is a schematiccross-sectional view of a second chip package structure according to thesecond embodiment of the present invention. As shown in FIG. 6, the topsurface of the first encapsulant is at a level lower than the bottomsurface 224 b of the circuit substrate 220. The heat-conductiveinterlayer 270 is located on the top surface of the first encapsulant240 a, a portion of the sidewalls of the cavity 230 a and between theheat sink 230 and the circuit substrate 220.

FIGS. 7A and 7B are graphs obtained from a simulation showing the degreeof warping between the chip package of the present invention and theconventional chip package at different temperatures. The simulatedtemperature is −65° C. in FIG. 7A and the simulated temperature is 225°C. in FIG. 7B. In addition, the curve 710 represents the warping of thediagonal line in a conventional chip package. Similarly, the curve 720represents the warping of the diagonal line in the chip packagestructure shown in FIG. 2G and the curve 730 represents the warping ofthe diagonal line in the chip package structure shown in FIG. 5. Asshown in FIGS. 7A and 7B, there are substantial improvements in thedegree of warping for the chip package structure fabricated according tothe present invention.

In summary, the main idea behind the chip package structure and the chippackaging process is to form a cavity on a heat sink, depositing amaterial into the cavity to form a first encapsulant and disposing achip over the first encapsulant. The first encapsulant serves toincrease the gap formed between the chip and the heat sink. Thereafter,the encapsulating material is deposited to form a second encapsulantthat encapsulates the chip. The first encapsulant and the secondencapsulant are fabricated from an identical material such as epoxyresin so that they have an identical coefficient of thermal expansion.Hence, the degree of warping in the chip package is minimized when thesecond encapsulant is cooled. Furthermore, since the chip is entirelyencapsulated by the first encapsulant and the second encapsulant, auniform thermal stress is generated around the chip. As a result, thechip and the conductive wires are able to remain in a fixed location andprovide the chip packaging process with a higher overall yield.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the structure of the presentinvention without departing from the scope or spirit of the invention.In view of the foregoing, it is intended that the present inventioncover modifications and variations of this invention provided they fallwithin the scope of the following claims and their equivalents.

1. A chip packaging process, comprising: forming a cavity on a heatsink; forming a first encapsulant at the bottom of the cavity; disposinga circuit substrate on the heat sink, wherein the circuit substrate hasan opening that corresponds in position to the cavity; disposing a chipon the first encapsulant; electrically connecting the chip and thecircuit substrate; forming a second encapsulant over the firstencapsulant to encapsulate the chip and a portion of the circuitsubstrate; and forming a plurality of conductive bumps on the circuitsubstrate.
 2. The chip packaging process of claim 1, further comprisingforming a heat-conductive interlayer over the first encapsulant and theheat sink before disposing the chip.
 3. The chip packaging process ofclaim 2, wherein the heat-conductive interlayer is a metallic layer. 4.The chip packaging process of claim 2, wherein the heat-conductiveinterlayer is a patterned metallic layer.
 5. The chip packaging processof claim 1, wherein the step of electrically connecting the chip and thecircuit substrate comprises performing a wire-bonding process.